1. Field of the Invention
The present invention relates to developing method and device used in an image forming apparatus for visualizing an electrostatic latent image formed on a photosensitive body or photoreceptor by applying toner to the latent image.
2. Description of the Related Art
Electrophotography has been used extensively in electronic copier, laser beam printer, facsimile, etc. as a process of printing an image on a sheet of plain paper. Electrophotographic development can be roughly grouped into wet development and dry development, and generally the latter dry development system is more popularly used. The dry development system can be further grouped into two-component development and monocomponent development systems, of which the latter monocomponent development is prevailing predominantly in recent years as the developing system mainly for small-size printers and copiers. The monocomponent development system is characterized in that the developing agent for use contains no carrier (such as iron or ferrite powder) and the developing device of this type can be constructed more simply than that of the two-component type, thus offering advantages in terms of compactness and cost over the two-component type developing device.
As an example of such monocomponent development system, impression development is known in the art. This system is characterized in that toner particles or toner carriers are brought into contact with electrostatic latent image on a photosensitive drum, with the relative peripheral velocity therebetween being held substantially zero (see U.S. Pat. No. 3,152,012, U.S. Pat. No. 3,731,148, Japanese Patent Application KOKAI Publication No. S47-13088 and Japanese Patent Application KOKAI Publication No. S47-13089). This system dispensing with magnetic material has many advantages in that the developing device and the image forming apparatus can be constructed simpler in structure and smaller in size and also it makes easier to use color toners.
Though the developing device of monocomponent type is thus advantageous in terms of simplification and compactness of the image forming apparatus, because it has no carrier for charging the toner and transferring the charged toner to a developing roller, the developing system is inferior to two-component developing system in respect of toner chargeability and toner transfer capability (or toner transfer amount). Reduced charge amount or quantity of toner on the developing roller and reduced toner transfer amount must be avoided, because these will cause reduced transfer amount of toner to electrostatic latent image, thereby causing reduction in image density and irregular development and hence formation of poor image.
In order to solve these problems, Japanese Pat. Application KOKAI Publication No. H5-289483 discloses a developing device, in which an optical toner layer detector is disposed upstream of developing region and adjacent to and in opposing relation to the developing roller. In this developing device, toner carry amount on the developing roller is detected by the toner layer detector and controlling is done in such a way that the pressure of a toner layer forming member on the developing roller is adjusted according to the detected value so that toner carry amount on the developing roller is maintained substantially constant. In this system, however, the disposition of the toner layer detector in opposing relation to the developing roller increases the manufacturing cost and the size of the developing device, thus this system being contrary to the compactness feature of the monocomponent developing system.
Furthermore, Japanese Pat. Application KOKAI Publication No. 2001-134093 discloses a developing device in which a toner layer forming blade is made of an electrically conductive material such as metal and, with a toner layer formed on the developing roller, the toner carry amount on the developing roller is detected by measuring electrical current flowing to the toner layer forming blade.
However, since the developing roller and the toner layer forming blade are disposed in contact with each other under the predetermined pressure in the monocomponent developing system, toner filming tends to occur frequently on the developing roller. In such a case, the electrical resistance of the developing roller surface is increased and the value of current flowing to the toner layer forming blade is varied even if the toner carry amount on the developing roller is the same as in the initial state, with the result that maintaining constant the toner carry amount is difficult.
It is an object of the present invention to provide an image forming apparatus, and a developing device and method used in an image forming apparatus, which can make possible maintaining stabilized image by accurately controlling the toner carry amount, in order to solve the above-described problems.
In order to achieve this object, the present invention provides a developing device in an image forming apparatus of monocomponent development type, wherein a developing roller is supplied with toner from a toner feed roller to form thereon a toner layer through application of a bias voltage to a toner layer forming member, while said toner layer forming member is pressed against said developing roller, comprising: a toner recovery system for recovering toner from said developing roller; a current variation detector for detecting variations in current flowing through said toner layer forming member applied with the bias voltage when feeding toner, during image forming operation, onto said developing roller from which the toner has been recovered by said toner recovery system; and a toner carry amount controller for controlling an amount of toner carried onto said developing roller on the basis of said variations in current.
In this case, the current variation detector may comprise a current detector for detecting current flowing through said toner layer forming member, a timer for counting time, and computing means for determining a difference in value of currents detected by said current detector before and after a predetermined time counted by said timer. Furthermore, the above predetermined time may be counted from the beginning of rotation of said developing roller.
Furthermore, in the developing device provided in the image forming apparatus according to the present invention, the current variation detector is operable to detect variations in current for each of first and second predetermined times having different lengths as counted from a beginning of rotation of said developing roller, and the toner carry amount controller is operable to control said toner carry amount on the basis of said detected current variation amounts for said first and second predetermined times. Furthermore, the toner carry amount controller may have tables of data of control quantities corresponding to the respective current variation amounts for said first and second predetermined times and is operable to control said toner carry amount on the basis of the control quantities found from said tables.
Furthermore, in the developing device in an image forming apparatus according to the present invention, the toner carry amount controller may operate to change the bias voltage for application to said toner layer forming member on the basis of said current variation. Alternatively, the toner carry amount controller may be operable to change the rotational speed of said toner feed roller on the basis of said current variation.
Still further, in the developing device provided in the image forming apparatus according to the present invention, the toner carry amount controller may be characterized in that it is operable so as to increase the toner carry amount if said controller determines on the basis of said variations in current that an amount of toner carried to said developing roller is at or below a predetermined amount, or under a predetermined amount.
The present invention further provides a developing method in an image forming apparatus of monocomponent development type, wherein a developing roller is supplied with toner from a toner feed roller to form thereon a toner layer through application of a bias voltage to a toner layer forming member, while said toner layer forming member is pressed against said developing roller, comprising the steps of: recovering toner from said developing roller by said toner feed roller at the end of image forming operation of said image forming apparatus; detecting variations in current flowing through said toner layer forming member during re-supply of toner to said developing roller at the start of image forming operation of said image forming apparatus; and controlling an amount of toner carried onto said developing roller by said toner feed roller on the basis of said variations in current.
The present invention still further provides an image forming apparatus comprising: a photoreceptor on which a latent image is formed; a developing roller for visualizing the latent image by feeding toner to said photoreceptor; a toner feed roller for feeding toner to said developing roller; a toner layer forming member applied with a bias voltage and pressed against said developing roller to form a toner layer on said developing roller; a toner recovery system for recovering toner on said developing roller; a current variation detector for detecting variations in current flowing through said toner layer forming member applied with the bias voltage when feeding toner, during image forming operation, onto said developing roller from which the toner has been recovered by said toner recovery system; and a toner carry amount controller for controlling an amount of toner carried onto said developing roller on the basis of said variations in current.